As a wire-saving transmission method of sending a plurality of signals by using less electric wires, a method of performing A/D conversion to convert an analog signal into a digital signal and sending the same together with an original digital signal in a mixed manner has been known as disclosed in Patent Document 1.
Alternatively, the method is realized by superimposing a modulated digital signal onto an analog signal of 4 to 20 mA by using a frequency shift modulation as disclosed in Patent Document 2, or the like.
What is important in these transmission methods of performing the A/D conversion to transmit the analog signal is a resolution which is greatly related to the accuracy of the A/D conversion. As the number of bits used for the transmission is increased to transmit one analog signal by using a plurality of digital signals such as 10 or 24 signals, the accuracy and resolution will enhance. However, the signal transmission efficiency will be deteriorated accordingly. In addition, when the transmission is performed in a mixed manner together with the transmission of the digital signals, the transmission amount of the digital signals decreases accordingly.
Thus, in the A/D conversion performed on the analog signal to be accurately transmitted, as the number of digital signals converted from the analog signal is increased, the amount of information converted into digital signals and transmission delay will increase.
Moreover, since the system, which superimposes the digital signal on the analog signal of 4 to 20 mA by using the frequency shift modulation to transmit the same, is based on a method that modulates the digital signal and puts the same on a common analog signal functioning as a basis, only one type of analog signal can be transmitted.